1) Field of the Invention
The present invention relates to a method for correcting or enhancing a contour of a color image, in which a contour correction signal in either or both of the vertical and horizontal directions are produced from image signals appeared in a plurality of channels; and the contour correction signal is added to the image signals of the respective channels to correct the contour of the color image.
2) Prior Art Statement
Hitherto, there have been known various methods for correcting the contour of a color image such as an out-of-green, a three-channel method and a brightness method. In the out of green method, the contour signal is derived from a green color signal appearing in a green color signal channel and the contour signal thus derived from the green color signal is added to image signals in respective channels of red, blue and green as a contour correction signal; in the three-channel method, contour signals are extracted from red, green and blue color signals appearing on the respective red, green and blue channels and the contour signals are added to the respective color image signals as contour correction signals; and in the brightness method at first a brightness signal is produced by mixing color image signals, a contour signal is derived from the brightness signal, and then the contour signal is superimposed on the color image signals in the respective color channels as a contour correction signal.
The above-mentioned three channel contour correcting method is disclosed in, for instance, "HOSO GIJUTU" issued on December, 1982, Vol. 35, No. 12, pp 77; and the brightness contour correcting method in Japanese Patent Preliminarily Publication Kokai Sho No. 63-90283.
In the above-mentioned out-of-green contour correcting method, the contour correction signal is produced solely from the green color signal. Since the green color signal has a high SN ratio in itself, the contour correction signal produced from the green color signal also has a high SN ratio. However, when an image of a monochromatic object is picked up or when an image of an object is picked up under monochromatic illumination, that is to say, when there is no output in the green color channel, the contour correction signal can not be obtained, so that the displayed image can not have a sharp contour.
On the other hand, in the conventional three-channel method for an correcting the contour of image, there is a drawback in that the SN ratio is aggravated. Particularly, the SN ratio of a frequency component less than 1 MHz is remarkably decreased.
Further, in the known brightness method for correcting the contour of an image, since one contour correction signal generating circuit is commonly used in a plurality of color channels, it is impossible to give the best correction of the contour in each of the color channels. For instance, in the conventional color television camera, the sensitivity of the blue channel is lower than the sensitivities of the other red and green channels by about two or three times. Therefore, when such a television camera is used in a studio where the color temperature is generally set to about 3000 K., it is necessary to increase the gain of the blue channel of the television camera. Thus, when extracting the contour signals from the color signals, it is preferred to increase the slice level, at which noise in the signal is cut off, in the blue channel so as to be larger than those in the other channels. However, in the brightness method, it is impossible to increase the slice level of the blue channel only.